A diffraction grating is a collection of reflecting or transmitting elements separated by a distance comparable to a wavelength of light. It may be thought of as a collection of diffracting elements, such as a pattern of transparent slits or apertures in an opaque screen, or a collection of reflecting grooves on a substrate. A reflection diffraction grating consists of reflecting grating elements disposed on a surface, whereas a transmission diffraction grating consists of transmitting grating elements disposed on a transparent surface or within a transparent slab of material. Light incident on a diffraction grating will diffract and, upon diffraction, will have an electric field amplitude or phase, or both, modified in a predictable manner, so that light at different wavelengths will propagate at different directions. This wavelength separating property of a diffraction grating is used to build spectrometers, wavelength selective optical switches, ultrashort laser pulse compressors, and other wavelength-selective optical devices.
In optical devices based on transmission diffraction gratings, the optical paths of incident and diffracted light are separated. This allows for a greater freedom in positioning of optical elements used in redirecting and collimating the incident and the diffracted light beams. Today's transmission diffraction gratings are usually free of absorptive materials such as metals, which reduces light absorption and makes the gratings usable in high-power laser applications. Furthermore, modern transmission diffraction gratings can be made entirely out of inorganic materials. Lack of organic materials results in improved environmental stability and reliability of the gratings. High environmental stability is particularly important in wavelength-selective optical switch applications of the gratings. Most of the transmission gratings based on inorganic materials are surface-relief gratings, in which the transmitting grating elements are transparent grating lines or grooves running parallel to each other on a grating surface. In a high-efficiency transmission diffraction grating, the grating lines or grooves modify only the optical phase distribution of an incoming light wave, whereby high diffraction efficiency becomes achievable. Herein, the term “diffraction efficiency” is defined as a proportion of the optical power of the diffracted light in the optical power of the incoming light.
A high-efficiency surface-relief transmission diffraction grating can be obtained by etching a groove pattern in a transparent overlayer on a glass or fused silica substrate, or in the substrate itself. For example, U.S. Pat. No. 5,907,436 by Perry et al. discloses a transmission diffraction grating obtained by etching a binary groove pattern in a top layer of a multilayer dielectric stack deposited on a transparent substrate. Further, U.S. Pat. No. 7,142,363 by Sato et al. teaches a transmission diffraction grating obtained by etching a trapezoidal groove pattern in a bilayer dielectric stack deposited on a transparent substrate. Both diffraction gratings of Perry and Sato provide a high diffraction efficiency at an oblique angle of incidence, the grating period being smaller than the wavelength of incoming light, but larger than one half of the wavelength of the incoming light. At these conditions sometimes referred to as “Bragg conditions”, only one diffraction direction, or non-zero diffraction order, is possible. A high efficiency of diffraction in that order can be achieved by properly selecting the diffraction grating period and duty cycle of the grating lines.
Referring to FIG. 1, a prior-art surface-relief bilayer transmission diffraction grating 10 is shown in a cross-sectional view. The grating 10 has a grating layer 13 consisting of grating lines 14 formed on a transparent substrate 12. An antireflective layer 15 consists of antireflective lines 16 formed on top of the grating lines 14. Both the grating lines 14 and the antireflection lines 16 have rectangular cross-sections. In other words, the grating 10 has a so-called binary profile. The rectangular cross-sections are typically achieved by selecting a highly anisotropic etch process for etching through the antireflective layer 15, followed by etching through the grating layer 13 using another highly anisotropic etch process. A diffraction efficiency modeling is used to determine a duty cycle of the grating structure, defined as a ratio of the air gap to the grating period, that gives maximum diffraction efficiency at a given grating line density. For example, to achieve a diffraction efficiency of >90% within a so-called telecommunications C band having a wavelength range of 1.52 to 1.56 microns, a grating structure of FIG. 1 having silicon dioxide (SiO2) antireflective layer 15 and tantalum oxide (Ta2O5) grating layer 13 should have a grating line density of approximately 900 lines per mm and a duty cycle of about 0.44. A different duty cycle will be required for a different value of grating line density.
Although efficient and reliable, surface-relief transmission diffraction gratings of the prior art have not yet found a widespread use due to their high cost. The high cost is related to the multitude of the process steps used in manufacturing transmission diffraction gratings, as well as to difficulties associated with maintaining high diffraction efficiency and low polarization dependence for different grating batches, as well as across a single grating; the latter is especially true for gratings having large surface area. The manufacturing difficulties result from sensitivity of the diffraction efficiency of the transmission surface relief diffraction gratings to the manufacturing process tolerances. Even at tight tolerances, parameters of the grating lines may vary, which leads to variations of the diffraction efficiency.
It is an object of the present invention to provide a low-cost surface-relief transmission diffraction grating that can be manufactured using a reduced number of process steps. Advantageously, a diffraction grating of the present invention has a high enough diffraction efficiency, so that the requirements for process tolerances can be relaxed, resulting in further cost reduction. It is another object of the present invention to provide a corresponding etch mask parameters selection method and a general manufacturing method of a transmission diffraction grating.